Kinetics of uptake and elimination of silicic acid by a human subject: a novel application of 32Si and accelerator mass spectrometry.

Silicon is possibly important in human physiology in protecting against the toxic effects of aluminium, but the kinetics of uptake and excretion of silicic acid, the bioavailable form, are not well characterised. We have used 32Si as a tracer in a human uptake experiment to determine a gastrointestinal uptake factor for silicic acid, and to elucidate the kinetics of renal elimination. Urine collections were made for extending intervals from 2 to 12 h over 2 days following ingestion by a single human subject of a neutral silicic acid solution containing tracer levels of 32Si (t1/2 approximately 150 y). Silicon was isolated as SiO2 and the 32Si content determined by accelerator mass spectrometry (AMS), using a gas-filled magnet technique to eliminate a prolific isobaric interference from 32S. Silicon uptake appears to have been essentially complete within 2 h of ingestion. Elimination occurred by two simultaneous first-order processes with half-lives of 2.7 and 11.3 h, representing around 90% and 10%, respectively, of the total output. The rapidly eliminated 32Si was probably retained in the extracellular fluid volume, whilst the slower component may represent intracellular uptake and release. Elimination of absorbed 32Si was essentially complete after 48 h and was equivalent to 36% of the ingested dose. This establishes only a lower limit for gastrointestinal absorption as, although there was no evidence for longer term retention of additional 32Si, the possibility could not be excluded by these results.

Determination of aluminium-26 in biological materials by accelerator mass spectrometry.

Studies of the biological chemistry of aluminium can gain significantly from the use of the long-lived isotope 26Al as a tracer, although the cost of the isotope often precludes its determination by radiochemical counting techniques. Accelerator mass spectrometry (AMS) provides an ultra-sensitive method of determination, free from isobaric interference from atomic (26Mg) or molecular species. The source materials for AMS can be aluminium oxide or phosphate, both of which can be readily prepared at a sufficient level of purity from biological substrates. Natural aluminium (27Al, 100%) is added to the preparations as a chemical yield monitor and to provide the reference for the isotope ratio measurement. 26Al/27Al ratios can be determined over the range 10(-14)-10(-7), implying a limit of detection for 26Al of around 10(-18) g. The precision of measurement and long-term reproducibility are < 5% and < 7% (RSD), respectively. Chemical methodologies for routine measurements on blood and urine samples have been developed.